The present invention relates generally to devices and methods for detection of analytes in test samples. More specifically, the present invention provides solid phase test strip devices and methods that combine an internal indicator on the test strip with an external mark located on a support.
Various analytical procedures and devices are commonly employed in detection assays to determine the presence and/or amount of substance of interest or clinical significance which may be present in biological or non-biological fluids. Such substances are generally termed “analytes” and can include antibodies, antigens, drugs, or hormones.
The present invention includes lateral flow chromatography assay formats. Generally, these assays have an extended base layer on which a differentiation can be made between a sample application region and an evaluation region. In typical use, the sample is applied to the sample application region, flows along a liquid transport path which runs parallel to the base layer, and then flows into the evaluation region. A capture reagent is present in the evaluation region, and the captured analyte can be detected by a variety of protocols to detect visible moieties associated with the captured analyte. For example, the assay may produce a visual signal, such as color change, fluorescence, luminescence, and the like, when indicating the presence or absence of an analyte in a biological sample.
Optimally, such test devices will provide a characteristic signal when the analyte is present in a sample, and a different signal when the analyte is absent from a sample. Most typically, the test device will display a “plus” (+) signal in the presence of analyte, and a “minus” (−) signal in the absence of analyte. The plus/minus test result format has enjoyed enthusiastic customer response and wide commercial success.
Test devices of this kind are well known in the art, and play an important role in areas such as clinical chemistry. They are used by skilled clinicians and lay person alike. Thus, there is a strong impetus to provide devices that are simple and reliable. Desirably, the assays are single-step devices wherein the user need only apply the sample prior to viewing the result. Single-step devices obviate the necessity of performing complicated and time consuming processing steps that may introduce errors in the end result.
Examples of such assays include pregnancy tests, ovulation tests, various urine, saliva, spinal, and blood tests, as well as other clinical or diagnostic assays.
Test devices typically use materials that specifically bind with an analyte of interest. A homologous pair of specific binding pair members (“sbp members”), usually an immunological pair comprising a ligand and a receptor (antiligand), is involved, wherein one of the sbp members is labeled with a label that provides a detectable signal. The immunoassay methodology results in a distribution of the signal label between signal label bound in a complex of the sbp members and unbound signal label. The differentiation between bound and unbound signal label can be a result of physical separation of bound from unbound signal label or modulation of the detectable signal between bound and unbound signal label.
In developing an assay device, there are many considerations. One consideration is to provide substantial differentiation between the observed signal resulting from signal label when bound as compared to unbound. Another consideration is the ease with which the observed signal can be detected and serve to differentiate between the presence or absence of analyte of interest. Other factors include the precision with which the test devices must be manufactured. Therefore, in developing an assay that can be used by untrained personnel, such as assays to be performed in the home, medical offices and the like, the technique for performing the assay should be simple, and the method of manufacturing the assay should be straightforward.
Plus/Minus Assays
Of particular interest to the present invention are test devices of the type described in U.S. Pat. No. 5,145,789 to Corti et al., the disclosure of which is incorporated herein by reference. Corti et al. discuss a built-in positive control to indicate successful operation of a pregnancy test device. The positive control is envisaged as a horizontal tract that always stains, independent of the presence of hCG in the urine, and is described as an area on a membrane that contains immobilized hCG. Regardless of whether hCG is present in the biological sample, it is intended that during operation, the upstream mobile labeled hCG binding reagents will always bind to the immobilized hCG, thereby forming a horizontal line, or minus sign, in the reading area.
A similar approach for providing a minus sign in a test device is described in U.S. Pat. Nos. 4,916,056 and 5,008,080 to Brown, III et al., the disclosures of which are incorporated herein by reference. As illustrated, the positive control is formed by providing a binding substance within the test strip matrix, and is formed in the shape of a rectangular bar, or minus sign. The binding substance of the minus sign is intended to bind the labeled material regardless of the presence or absence of the analyte of interest in the test sample.
Another approach for providing a positive control in a test device is described in EP Patent Publication No. 0 249 418 to Graham, Jr., the disclosure of which is incorporated herein by reference. As described, the control zone has anti-human IgG or IgM immobilized thereon, for nonspecifically capturing human immunoglobulin ubiquitously present in all similar human aqueous samples. The immobilized antibody is intended to provide a signal in a “minus” pattern, regardless of the presence or absence of the analyte of interest in the test sample.
Osikowicz et al., in U.S. Pat. No. 5,075,078, describe yet another approach for providing a positive control in a plus/minus test device. The positive control is disposed on a test strip in a rectangular bar configuration. The control bar is oriented on the strip so that it lies neither perpendicular nor parallel to the direction of fluid flow, but rather lies at an intermediate orientation, i.e., at a 45 degree angle.
Still yet another approach for providing a positive control in a test device is provided in U.S. Pat. No. 5,401,667 to Koike. As described, the test device provides a plus/minus format, but considers alternative geometric symbols as well. A portion of the chromatographic medium is removed, or otherwise partially blocked, thereby affecting the flow path of the liquid. It is suggested that this modification enhances the signal of the device.
Wong et al., in EP Patent No. 0 260 965, describe another test device that utilizes the plus/minus format. Wong et al. discuss a multiple-step diagnostic assay with a horizontal positive control line sprayed onto a test membrane.
The previous methods discussed above accomplish the “appearance” of a minus sign (−) by placing an indicator (positive control) line perpendicular to the test line, directly onto the test strip. Typically, the control line develops with any sample flow, while the test line develops only with a positive sample flow. Thus, the previous assays involve a control mechanism inherent to the matrix membrane test strip, and require a specific manufacturing step to apply the control line to the strip.
Other previous devices display a printed minus sign positioned on the matrix and across the test line. These devices typically incorporate a positive control line downstream from, and parallel to, the test line. Such devices are limited as the test strips may present a line that is visible before the sample is added.
Previous methods are further disadvantaged as the additional manufacturing step involves a difficult placement procedure to orient the perpendicular line directly in the center of the viewing window. Whether the perpendicular line is a printed minus sign, or a reagent-based control line, this approach is particularly ill suited for certain matrix construction procedures, including web processing methods that involve a continuous flow or continuous roll application approach.
Therefore, it would be desirable to provide a test device that does not require this extra processing step of depositing a perpendicular line onto the test strip, or does not leave a line that is visible before the sample is added to the device. This invention fulfills these and other needs.
Transparent Membranes
The use of transparent test strips in diagnostic assays is known in the art. In U.S. Pat. No. 4,824,640, Hildebrand et al. discuss a transparent reagent carrier layer suitable for evaluation by transmission photometry. As described, the transparent nature of the film of plastic provides a suitable carrier material as compared to opaque films.
The use of a transparent test strip is also discussed in U.S. Pat. No. 5,110,550 to Schlipfenbacher et al. As described, this test device includes a white non-transparent covering layer situated above a color-forming layer. During operation of the test device, the covering layer becomes transparent in the moist state. Through the transparent covering layer, the user is then able to observe any reaction occurring in the color-forming layer below.
The use of a clearing agent in an immunochromatographic assay is discussed in U.S. Pat. No. 6,165,798 to Brooks. As described, the test strip membrane is rendered. transparent by wetting the membrane with a clearing agent, thus reducing the amount of light scattered by the membrane fibers.
In U.S. Pat. No. 6,187,268, Albarella et al. describe a transparent flow through membrane for use in test devices, but do not suggest a control feature to indicate a positive or negative test result. The membrane described in Albarella et al. is not configured to become transparent only when wet. The membrane is transparent whether wet or dry.
While conceivably workable in some circumstances, the previous detection systems that employ transparent membranes are of limited utility. There is no teaching or suggestion in current art for a test device with a transparent membrane that utilizes a control feature to indicate a positive or negative test result as provided by a mark on the underlying support.
In view of the foregoing, there remains a need in the art for a simple, efficient method for adding a positive control to a solid phase assay that does not require the manufacturing step of fixing a positive control binding member to the assay test strip, and that does not leave a substantially visible before the sample is added to the device. It would further be desirable to achieve improved test device formats that incorporate transparent membranes as part of a control or display feature.
Additionally, the assay of the present invention should overcome the disadvantages described above in connection with the previous test device systems.